Dude, I saw a new piece of land in another screenshot from a convention where they held a 2 mission demo involving the new helicopter and parachute. Looks like Staten Island. Don't know if it's just for 1 mission or if we're really getting a 5th piece to GTA 4.

And, on a picture of the demo, there are also rumours that the TV shows Staten Island (or other) because, I've never seen such road with these trees.